Extrapolation schemes of key comparison results in gas analysis

Abstract

The GAWG (Gas Analysis Working Group of the Consultative Committee for Amount of Substance: Metrology in Chemistry and Biology) has developed an extrapolation scheme to transfer so that the expanded uncertainty as reported in a key comparison to other amount fractions. Typically, such a comparison only addresses one amount fraction level, whereas the relevant service of a national metrology institute usually covers an interval. In this work, we show how, based on empirical evidence and the uncertainty reported in a key comparison, the measurement uncertainty can be predicted over a wide interval. We do so for four components: carbon monoxide, carbon dioxide, propane and methane, and for two matrices, nitrogen and air. The empirical results confirm to a large extent the validity of the extrapolation scheme of the GAWG above 10 μmol mol −1 . At the same time, our results show that predictions at the ends of the intervals are often providing a conservative value for the expanded uncertainty. We also conclude that the GAWG extrapolation scheme is less suited for CMCs where additional effects (e.g., adsorptivity, reactivity) play a role. Our extrapolation scheme is more flexible and better suited for this purpose. We also note that the GAWG does not perform well if the nominal value of the amount fraction in the key comparison is substantially below 10 μmol mol −1 .